This invention relates to antennas and antenna units used for mobile communication systems.
An example of prior art microstrip antenna, for use in a mobile communication system such as a car radio, is shown generally at 35 in FIGS. 6A and 6B wherein numeral 31 indicates a dielectric base plate with a patch electrode 32 and a shielding electrode 33 formed on its surfaces. A connector 34 with an inner conductor and an outer conductor is attached to the same side of the base plate 31 as the shielding electrode 33, with the inner conductor connected to a feed point 32a of the patch electrode 32 and the outer conductor connected to the shielding electrode 33. Electromagnetic waves are received and transmitted through the patch electrode 32 such that the functions of an antenna can be carried out.
If one attempts to reduce the outer dimensions of the base plate 31 in order to produce a compact microstrip antenna, however, its antenna characteristics will be adversely affected. For this reason, it was not possible as a practical matter to reduce the length of the patch electrode to less than one-tenth of the wavelength. Because the connector 34 protrudes from the bottom surface, furthermore, the overall height of the microstrip antenna 35 could not be reduced beyond a certain limit for easy surface-mounting of the antenna.